And herein lies your problem. Because the surface of this boulder is SO irregular, it will be extremely difficult to make any liner attachment to the boulder Leak-Free.

It would be so much easier to lift the boulder, place the liner beneath it and reset the boulder more perpendicular to the ground. Another factor to consider is, if you are going to drill a hole(s) for piping, you will need the proper equipment or sub it out to a concrete contractor.